Why does my softbody have a hole in the bottom?
If by 'hole' you mean the cavity that has formed under the sphere where the bottom surface has become concave, then that is a limitation of using Soft Body simulation to simulate what you consider to be a solid.
The Soft Body simulation in your example is simulating the object's surface only. This means that there is no internal structure to push the deformed surface back out. The surface is made up of vertices and edges and the simulation tries to move the vertices so as to get the edges back to their preferred lengths and relative angles.
In your case, the deformation has pushed the bottom of the sphere beyond flat and it has effectively folded back on itself. Since there is no internal structure to resist that deformation it has 'flipped' inside out and found a new 'at rest' position - where the inverted geometry can reach the edge's preferred lengths.
One solution is to add internal geometry to allow the simulation to 'push' those surfaces back out again. One way of achieving this is to criss-cross the inside of the mesh with &quot;struts&quot; of edges which can oppose the deformation and act as internal material pushing back out.
To achieve this, go into Edit mode,select a vertex and Shift-select the opposite vertex, then hit F. This will create an edge spanning the middle of the sphere. Repeat this for vertices in many opposing directions (keep the spread even so that the internal &quot;pressure&quot; is evenly distributed.
Now running your simulation should have the soft body reacting as if it has internal structure.